Modeling 400–500-kyr Pleistocene carbon isotope cyclicity through variations in the dissolved organic carbon pool.

The carbon isotope (δ 13 C) record from the Plio-Pleistocene shows prominent 400-kyr cycles with maximum values at eccentricity minima during the Pliocene. The period extends to 500 kyr in the Pleistocene after ~ 1.6 Ma. Five δ 13 C maxima occurred at ~ 0.2, 0.5, 1.0, 1.5 and 1.9 Ma over the last 2 Ma. Although several hypotheses have been suggested to explain why the 400–500-kyr cycles are so strong in δ 13 C records and how they may have originated, the mechanism is still not clear. The aim of this study was to test the dissolved organic carbon (DOC) hypothesis, which was proposed recently to explain this 400–500-kyr cycle in deeper time. We used an intermediate complexity box model that is computationally efficient for studies involving longer timescales. The model incorporates sophisticated microbial processes, dividing the oceanic carbon cycle into a rapid and a slow cycle. The model result suggests that when more nutrients enter the surface ocean, the rapid carbon cycle is more active, and less refractory DOC (RDOC) is produced. The opposite sequence occurs when fewer nutrients enter the ocean. The modeled RDOC concentration and the δ 13 C of dissolved inorganic carbon (DIC) are anti-correlated with riverine nutrient input. According to mass conservation, the release of isotopically lighter carbon from the RDOC pool leads to lighter DIC δ 13 C while an increase in the RDOC pool enriches it. The transient simulations produced a one-to-one correspondence between modeled and measured δ 13 C. This study supports the hypothesis that chemical weathering-induced variations in the DOC pool act as a pacemaker for δ 13 C changes over 400–500-kyr cycles. [ABSTRACT FROM AUTHOR]